1. Technical Field
The present invention relates to water soluble flux compositions and their use in processes for soldering members, particularly metallic members together. The flux compositions of the invention are particularly useful in the field of microelectronics, such as in the preparation of integrated circuit boards
2. Description of the Prior Art
A common task in the manufacture of microelectronic components involves the manufacture of single chip of multi-chip modules having input/output pins which are inserted into a substrate. The input/output pins provide the needed electrical connections to the integrated circuit chip or chips which are subsequently connected to the substrate or carrier. In other presently known manufacturing processes, a chip is soldered directly to a printed circuit board. With either process, solder flux compositions have typically been applied to the pins in order to connect the component to the selected substrate, for instance, the printed circuit board. Flux compositions are employed to remove oxides from the pins and to prevent the pins from oxidizing when subjected to elevated temperatures for soldering, thereby serving to maintain the electrical conductivity of the pins. Once the solder is applied, any flux composition or residue remaining on the pins and substrates must be removed to provide as clean a substrate as possible.
The soldering operation becomes increasingly difficult when applied to microelectronics. The pieces to be joined are extremely small, making cleaning, tinning, post cleaning and inspection difficult. In some cases, to avoid overheating, only the lead portion of the parts to be joined can be heated during the soldering operation. Cleaning and post cleaning are difficult due to the small size of the component, their large numbers, and the potential damage to the electronics by the cleaning solutions used. Another problem source results from the fact that many of the known soldering fluxes are corrosive. In the environment of microelectronics, corrosion from any residual flux can ruin an extremely costly device.
The provision of a non-toxic, non-corrosive soldering flux which is water cleanable holds many advantages for the microelectronics application. Although aqueous cleaning fluxes have been available in the past for use in solder fluxing prior to reflow, and in combination with pastes, these prior art compositions have not provided the desirable property of stickiness or tackiness needed to hold integrated circuit chips and other precisely placed components during reflow. Those prior art formulations which exhibited the desirable property of tackiness at room temperature generally exhibit large viscosity/fluidity end changes due to solder reflow temperatures. The solubility characteristics of the commercially available formulations in water has been less than optimum. This property is extremely important in microelectronic applications with low stand-off heights such as the 0.002-0.004 inch chip attach to substrate heights presently encountered.
As has been mentioned, many of the organic water soluble fluxes presently available contain corrosive materials such as halides. Very aggressive chemistry containing corrosive materials such as chlorides, bromides, strong bases such as triethanolamine, or easily ionized organic acids which produce a low pH aqueous solution, necessitates rapid and thorough flux removal after processing, before deterioration of circuitry and interconnections can occur. For instance, a flux composition which contains free halogen can result in conversion to hydroacids and corresponding halide ions by hydrolysis at the soldering temperature. Hydroacids can further react with organic materials present in the flux to free halide ions. Accordingly, if the flux residue is not entirely removed, it will lead to corrosion of the parts soldered.
Many of the commercially available low residue fluxes which we have tested have proved to be too thin, running out from under the chip during the manufacturing operation and failing to hold it in place.
The present invention has as its object to provide a water soluble soldering flux which possesses the required degree of tackiness to hold precisely aligned components during solder reflow.
Another object of the invention is to provide such a soldering flux which is environmentally superior, requiring no chemical reclamation or waste treatment.
Another object of the invention is to provide a soldering flux which provides the needed degree of oxide removal while forming relatively inert reaction products after reflow which do not necessitate immediate flux removal.
Another object is to provide a soldering flux having a tacky carrier medium which is completely water soluble, allowing it to be easily cleaned without the use of organic solvents.
Additional objects, features and advantages will be apparent in the written description which follows.